The invention is related to providing a bubble water depth measuring method for compensating for a total error of a bubble water depth or water level measurement in a reservoir that a water level shifts in a wider range to enable the measurement of the water depth or water level with a higher accuracy, using an industrial pressure transducer, in which the pressure transducer is manufactured in an environment that a circumferential air temperature is changed in the range of xe2x88x9240xc2x0 C. to +50xc2x0 C., and a system thereof.
Particularly, the invention is related to providing a bubble water depth measuring method for measuring a water column pressure to enable the remote-measurement of a water depth or water level in a reservoir, a lake, or a river. Herein, it is noted that only a water depth measurement now will be explained below, because a bubble water level meter is a device for measuring a water depth and calculating it into a water level.
There has been much interest in a bubble water level meter that can be used for a hydrology observatory, because the bubble water level meter can measure a water depth under an icy condition, if a water surface is frozen in a reservoir, a lake, or a river. The water level meter for the hydrology observatory have characteristics as follows:
In a reservoir where the changing depth of a water level is usually up to 10 m.
There are many areas where a circumferential air temperature of a water level observatory post is in the range of xe2x88x9240xc2x0 C. to +50xc2x0 C. in seasons.
Most of the water level observatory posts are not equipped with warming and cooling utilities but also a power source unit.
The demand for a remote water level measuring system is being increased because many water level observatory posts are established in uninhabited places.
In light of these facts, there is a problem in that a bubble type water level meter has a lower accuracy under the conditions of the severe weather change in seasons and a larger water level change depth.
The bubble type water level meter has advantages in that its installing, maintenance and operating costs are relatively lower and can a water level even when a reservoir and a river are frozen in winter, but it is broadly not used as a hydrology observatory because its measuring error is larger.
Also, the bubble type can be operated in more stability compared with other water level meters in a river where the concentration of floating particles is higher. A sand and earth layer is swiftly changed and saves on the maintenance and operating costs, but its measuring accuracy is low.
Referring to FIG. 1, error factors that are caused upon measuring a water depth according a bubble generating method will be explained below, and an unit of a water column pressure will be represented as cmH20 or mmH20 for the purpose of consulting the convenience in calling the term xe2x80x9cerrorxe2x80x9d as an abbreviate of xe2x80x9cabsolute errorxe2x80x9d. 1 is a water column pressure tube, which will be called xe2x80x9ca water depth measuring tubexe2x80x9d, 10 is a compressed gas generating device and 3 is a pressure transducer. A pressure applied to the lower end portion of the water depth measuring tube 1 is as follows:
Pc=xcex3hx+Pa,xe2x80x83xe2x80x83(1) 
Wherein, hx is an altitude difference of water filled in the water depth measuring tube 1, which is considered as a water depth. xcex3 is a specific gravity (gm/cm3) of water. xcex3hx is a water column pressure. Pa is an atmosphere pressure on a water surface.
The pressure transducer 3 measures a surplus pressure, not for an atmosphere pressure (P≈Pabxe2x88x92Pa; Pab-Absolute pressure). The water depth measuring tube 1 filled with water generates bubbles at the lower end portion with compressed gas being supplied to the upper end portion thereof. Upon generating of the bubbles, the pressure transducer 3 measures the compressed gas pressure, and its result is as follows:
Pmx=xcex3hx+xcex94px+(Paxe2x88x92Pao)xe2x88x92xcex3gHo+xcex94pb;xe2x80x83xe2x80x83(2) 
Wherein, xcex94px is an absolute error of the pressure transducer 3 at the time of measuring Pmx. xcex94Pa=(Paxe2x88x92Pao) is a difference between an atmosphere Pa on the water surface and an atmosphere Pao applied to the pressure transducer 3. Generally, Paoxe2x89xa0Pa because a water level observatory post is placed on a much higher position than a water surface. Ho is an altitude difference between the lower end portion of the water depth measuring tube 1 and the mounting position of the pressure transducer 3. xcex3g is a density of compressed gas to be supplied to the water depth measuring tube 1. xcex94pb is a pressure of a supplementary compressed gas changed according to the bubble pressure that is formed at the lower end portion of the water depth measuring tube 1. xcex94pb will be ignored because its reducing method is now developed. In expression (2), all items are a measuring error of a water column pressure xcex3hx except for the water column pressure xcex3hx. What the measuring errors are summed up is assumed as total absolute error xcexa3xcex94x of the water column pressure xcex3hx as follows:
xcexa3xcex94x=xcex94px+xcex94Paxe2x88x92xcex3gHo,xe2x80x83xe2x80x83(3) 
Wherein, xcex94Pa=Paxe2x88x92Pao=xcex3xcex1(Hoxe2x88x92hx), xcex3a is an air density, which can be seen as       γ    g    ≈            γ      go        ⁢                                        P            mx                    +                      P            a                                    P          a                    .      
xcex3go is a density of a compressed gas to be used at an atmosphere. Un the condition that a water depth is changed over 10 m in a reservoir, the altitude difference between the lower end portion of the water depth measuring tube 1 and the mounting position of the pressure transducer 3 usually becomes Hoxe2x89xa720 m.
Looking into xcex94Pa and xcex3gHo, if Ho=20 m=2000 cm, hx is changed in the range of 200 to 1000 cm, an air density xcex3xcex1=1.2xc2x710xe2x88x923 gm/cm3 and a water depth is measured using a compressed gas, the change of xcex94Pa is as follows:
xcex94Pa=1.2xc2x710xe2x88x923(2000-200)=2.16 gm/cm2≈2.16 cmH2O(if hx=200 cm) 
xcex94Pa=1.2xc2x710xe2x88x923(2000-200)=1.2 gm/cm2≈1.2 cmH2O(if hx=1000 cm) 
xcex94gHo=1.2xc2x710xe2x88x923xc2x71.2xc2x72000=2.88 gm/cm2≈2.9 cmH2O(if hx=200 cm) 
xcex94gHo=1.2xc2x710xe2x88x923xc2x72xc2x72000=4.8 gm/cm2≈4.8 cmH2O(if hx=1000 cm) 
Herein, it is known that when hx=200xcx9c1000 cm, the error changing range is 2.16-2.9≈xe2x88x920.74 cm to 1.2-4.8≈xe2x88x923.6 cm due to xcex94Paxe2x88x92xcex3gHo. If an allowance error of the water depth, water level is xc2x11 cm, the error component of xcex94Paxe2x88x92xcex3gHo cannot be ignored. Of course, xcex3a was ignored though it is changed according to a temperature.
The pressure transducer 3 includes a temperature compensation circuit for correcting the property that a pressure sensor mounted therein is changed according to a temperature. But the temperature compensation circuit is lacking of compensating for an error changed according to a temperature t and a pressure P to be measured, perfectly. An error of a curve xcex94p=f(t;P) always happens.
For example, when the pressure transducer (Model PTX1000) that secures a pressure error xcex4p (=xc2x10.25% fs) at room temperature (t≈20-24xc2x0 C.) measures a pressure of P=1000 cm H2O with being cooled at xe2x88x9210xc2x0 C., its measuring error xcex94p≅xe2x88x9231 cm H2O, even though its use temperature range is introduced as xe2x88x9240xc2x0 C.xcx9c+90xc2x0 C. More precisional pressure transducer (Model PDCR862) has a useable temperature range of xe2x88x9254xc2x0 C.xcx9c+125xc2x0 C. and a measuring error xcex4p of xc2x10.1% fs. Therefore, xcex94p=8 gm/cm2, and an absolute error hx is xcx9c8 cm. Of course, if only a gas temperature is changed without cooling or heating the pressure transducer on the whole, the error is reduced. But, when a bubble water level meter is installed outdoor, the pressure transducer is cooled or heated on the whole. Under the condition that a circumferential air temperature is changed in the range of xe2x88x9240xc2x0 C.xcx9c+50xc2x0 C., there doesn""t exist any pressure transducer for securing xcex94p less than xc2x11 gm/cm3 in full range to be measured.
Therefore, even though it is possible to compensate for the error xcexa3xcex94x caused at the time of measuring the water column pressure xcex3hx, perfectly, when the water depth hx is calculated into       hx    =                  γ        ⁢                  xe2x80x83                ⁢        hx                    γ        l              ,
knowing nothing about an average density xcex3 of water causes a corresponding error hxe2x80x2x as follows:             h      x      xe2x80x2        =                  γ        ⁢                  xe2x80x83                ⁢                  h          x                            γ        xe2x80x2              ;            δ      ⁢              xe2x80x83            ⁢              h        x        xe2x80x2              =                                        h            x            xe2x80x2                                h            x                          -        1            =                        (                                    γ                              γ                xe2x80x2                                      -            1                    )                =                  δ          γ                    
Therefore, a total error of the measurement of the water depth hx is as follows:                                                         h              x              xe2x80x2                        =                                                            γ                  ⁢                                      xe2x80x83                                    ⁢                                      h                    x                                                  +                                  ΣΔ                  x                                                            γ                xe2x80x2                                              ;                ⁢                  
                ⁢                              δ                                          h                xe2x80x2                            ⁢              x                                =                                                                      h                  x                  xe2x80x2                                                  h                  x                                            -              1                        =                                                            δ                  γ                                +                                                      ΣΔ                    x                                                        γ                    ⁢                                          xe2x80x83                                        ⁢                                          h                      x                                                                                  =                                                δ                  γ                                +                                  Σδ                  x                                                                                        (        4        )            
Wherein, an error xcex4hxe2x80x2x is a parameter that varies according to a circumferential temperature and a measuring range, but not integer. Due to it, it cannot be simply compensated. Therefore, the error inquired above is one that a bubble type water depth measuring method has. There has been known a bubble type water depth, water level meter for excluding or compensating for xcex4xcex3 and xcexa3xcex4x. Only, the bubble water measuring methods are disclosed in Publications as follows:
1) U.S. Pat. No. 5,791,187 issued on Aug. 11, 1998, which is titled xe2x80x9cLevel Measurement Method using Measurements of Water Column Pressure Thereofxe2x80x9d
2) Canadian Patent No. 2,171,801 on Jan. 23, 2000, which is titled xe2x80x9cLevel Measurement Method using Measurements of Water Column Pressure Thereofxe2x80x9d
3) Germany Patent DE19620656C2 issued on Mar. 2, 2000
4) Japanese Patent No. 2855423 issued on Feb. 10, 1999
5) South Korean Patent No. 185260 issued on Dec. 28, 1998
These known corresponding publications disclose a method of measuring a water depth hx, which will be explained with reference to FIG. 2. 2 is a water column pressure tube that has a shorter length than a water depth measuring tube 1 by xcex94h. A water pressure of the water column pressure tube 2 filled with water is as follows: xcex3h2=xcex3(h2xe2x88x92xcex94h). The water depth measuring tube 1 and the water column pressure measuring tube 2 are connected through valves 51 and 52 to a buffer tank 4. Compressed gas is supplied through a valve 50 to the buffer tank 4. 3 is a pressure transducer.
As a part for enhancing the accuracy of the water depth measurement the arrangement uses two measuring tubes 1 and 2 to measure a water specific gravity xcex3 that is changed according to a temperature and components of water and then divide a water column pressure xcex3hx by the water specific gravity xcex3. But, it is interested in that the separate measurement of the water specific gravity xcex3 is not necessary. If only the water depth hx is measured, it can be measured in a simple and effective expression as follows:                                           h            x                    =                                    Δ              ⁢                              xe2x80x83                            ⁢              h                                      1              -                                                                    P                    m2                                                        P                    mx                                                  ⁢                                  (                                      1                    -                                                                  γ                        ao                                            ⁢                                                                        T                          o                                                T                                            ⁢                      Δ                      ⁢                                              xe2x80x83                                            ⁢                      h                                                        )                                                                    ;                            (        5        )            
Wherein, Pm2 is a bubble pressure generated at the water column pressure measuring tube 2, xcex3ao is a specific gravity of air, and       γ    ao    ⁢            T      o        T    ⁢  Δ  ⁢      xe2x80x83    ⁢  h
is ignored because of a very little value. Assuming that xcexa3xcex94x, xcexa3xcex942=0, the expression (5) is as follows:       h    x    =                              P          mx                                      P            mx                    -                      P            m2                              ⁢      Δ      ⁢              xe2x80x83            ⁢      h        =                            γ          ⁢                      xe2x80x83                    ⁢                                    h              x                        ·            Δ                    ⁢                      xe2x80x83                    ⁢          h                                      γ            ⁢                          xe2x80x83                        ⁢                          h              x                                -                      γ            ⁢                          (                                                h                  x                                -                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  h                                            )                                          =                                    γ            ⁢                          xe2x80x83                        ⁢                          h              x                        ⁢            Δ            ⁢                          xe2x80x83                        ⁢            h                                γΔ            ⁢                          xe2x80x83                        ⁢            h                          =                  h          x                    
Herein, it is noted that the water specific gravity xcex3 is entirely excluded from the expression (5). It is because hx and h2 has a difference xcex94h therebetween, and it is seen as xcex3x=xcex32. But, because xcexa3xcex94xxe2x89xa00, xcexa3xcex942xe2x89xa00, a water depth hxe2x80x2x is as follows:                                                                         h                x                xe2x80x2                            =                              xe2x80x83                            ⁢                                                                                                                  γ                        ⁢                                                  xe2x80x83                                                ⁢                                                  h                          x                                                                    +                                              ΣΔ                        x                                                                                                            γ                        ⁢                                                  xe2x80x83                                                ⁢                                                  h                          x                                                                    +                                              ΣΔ                        x                                            -                                              γ                        ⁢                                                  xe2x80x83                                                ⁢                                                  h                          2                                                                    -                                              ΣΔ                        2                                                                              ·                  Δ                                ⁢                                  xe2x80x83                                ⁢                h                                                                                        =                              xe2x80x83                            ⁢                                                                    γ                    ⁢                                          xe2x80x83                                        ⁢                                                                  h                        x                                            ⁡                                              (                                                  1                          +                                                                                    ΣΔ                              x                                                                                      γ                              ⁢                                                              xe2x80x83                                                            ⁢                                                              h                                x                                                                                                                                    )                                                                                                  γΔ                    ⁢                                          xe2x80x83                                        ⁢                                          h                      ⁡                                              (                                                  1                          +                                                                                                                    ΣΔ                                x                                                            -                                                              ΣΔ                                2                                                                                                                    γΔ                              ⁢                                                              xe2x80x83                                                            ⁢                              h                                                                                                      )                                                                                            ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                h                                                                                        =                              xe2x80x83                            ⁢                                                h                  x                                ⁢                                  xe2x80x83                                ⁢                                                      (                                          1                      +                                                                        ΣΔ                          x                                                                          γ                          ⁢                                                      xe2x80x83                                                    ⁢                                                      h                            x                                                                                                                )                                                        (                                          1                      +                                                                                                    ΣΔ                            x                                                    -                                                      ΣΔ                            2                                                                                                    γΔ                          ⁢                                                      xe2x80x83                                                    ⁢                          h                                                                                      )                                                                                                          (        6        )            
Herein, assuming that                     ΣΔ        x            -              ΣΔ        2                    γΔ      ⁢              xe2x80x83            ⁢      h        ⁢       less than  less than     1.0  
is very little value, it is ignorable. The expression (6) is represented as follows:                                                                         h                x                xe2x80x2                            =                              xe2x80x83                            ⁢                                                                    h                    x                                    ⁡                                      (                                          1                      +                                                                        ΣΔ                          x                                                                          γ                          ⁢                                                      xe2x80x83                                                    ⁢                                                      h                            x                                                                                                                )                                                  ⁢                                  (                                      1                    -                                                                                            ΣΔ                          x                                                -                                                  ΣΔ                          2                                                                                            γΔ                        ⁢                                                  xe2x80x83                                                ⁢                        h                                                                              )                                                                                                        ≈                              xe2x80x83                            ⁢                                                h                  x                                ⁡                                  (                                      1                    +                                                                  ΣΔ                        x                                                                    γ                        ⁢                                                  xe2x80x83                                                ⁢                                                  h                          x                                                                                      -                                                                                            ΣΔ                          x                                                -                                                  ΣΔ                          2                                                                                            γΔ                        ⁢                                                  xe2x80x83                                                ⁢                        h                                                                              )                                                                                        (        7        )            
Therefore, an absolute error xcex94hxe2x80x2x that is resulted from the measurement of the water is as follows:                               γ          ⁢                      xe2x80x83                    ⁢                      Δ                                          h                xe2x80x2                            ⁢              x                                      =                  xe2x80x83                ⁢                                            ΣΔ              x                        -                                          (                                                      ΣΔ                    x                                    -                                      ΣΔ                    2                                                  )                            ⁢                              (                                                      h                    x                                                        Δ                    ⁢                                          xe2x80x83                                        ⁢                    h                                                  )                                              =                                        =                  xe2x80x83                ⁢                              Δ            ⁢                          xe2x80x83                        ⁢            px                    +                      Δ            ⁢                          xe2x80x83                        ⁢            Pax                    -                                    γ              gx                        ⁢                          H              o                                -                                                  xe2x80x83                ⁢                              [                                          Δ                ⁢                                  xe2x80x83                                ⁢                                  p                  x                                            +                              Δ                ⁢                                  xe2x80x83                                ⁢                                  P                  ax                                            -                                                γ                  gx                                ⁢                                  H                  o                                            -                              Δ                ⁢                                  xe2x80x83                                ⁢                                  p                  2                                            -                              Δ                ⁢                                  xe2x80x83                                ⁢                                  Pa                  2                                            +                                                γ                  g2                                ⁢                                  H                  o                                                      ]                    ⁢                                    h              x                                      Δ              ⁢                              xe2x80x83                            ⁢              h                                                              =                  xe2x80x83                ⁢                              Δ            ⁢                          xe2x80x83                        ⁢            px                    +                      Δ            ⁢                          xe2x80x83                        ⁢            Pax                    -                                    γ              g                        ⁢                          H              o                                -                                                  xe2x80x83                ⁢                                            [                                                Δ                  ⁢                                      xe2x80x83                                    ⁢                  px                                ⁢                                  xe2x80x83                                -                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                                      p                    2                                                  +                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  pa                                +                                                      (                                                                  γ                        g                                            -                                              γ                        gx                                                              )                                    ⁢                                      H                    o                                                              ]                        ⁢                                          h                x                                            Δ                ⁢                                  xe2x80x83                                ⁢                h                                              ;                    
Herein, considering that xcex94xcex94pa (=xcex94Paxxe2x88x92xcex94Pa2) and (xcex3g2xe2x88x92xcex3gx)Ho is a very little value, it is ignored. The resulting expression is as follows:       γΔ                  h        xe2x80x2            ⁢      x        ≈            Δ      px        -                  (                              Δ            px                    -                      Δ            p2                          )            ⁢                        h          x                          Δ          ⁢                      xe2x80x83                    ⁢          h                      +          Δ      ⁢              xe2x80x83            ⁢              P        ax              -                  γ        gx            ⁢              H        o            
Therefore, xcex94hxe2x80x2x is as follows:                               Δ          hx                ≅                              [                                          Δ                px                            +                                                (                                                            Δ                      p2                                        -                                          Δ                      px                                                        )                                ⁢                                  hx                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                    h                                                              +                              (                                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                                          P                      ax                                                        -                                                            γ                      gx                                        ⁢                                          H                      o                                                                      )                                      ]                    γ                                    (        8        )            
The conventional method excludes the water specific gravity xcex3, but it doesn""t compensate for errors xcex3px, xcex3p2 of the pressure transducers and an error component due to an altitude difference.                               (                                    Δ              ⁢                              xe2x80x83                            ⁢                              P                ax                                      -                                          γ                gx                            ⁢                              H                o                                              )                =                  xe2x80x83                ⁢                                            γ              ao                        ⁡                          (                                                H                  o                                -                                  h                  x                                            )                                -                                                    γ                ao                            ·                                                P                  mx                                                  P                  o                                                      ⁢                          H              o                                                                        xe2x80x83                ⁢                                            γ              ao                        ⁢                                          H                o                            ⁡                              (                                  1                  -                                                                                    P                        mx                                            +                                              P                        o                                                                                    P                      o                                                                      )                                              -                                    γ              ao                        ⁢                          h              x                                          
For these reasons, the conventional method doesn""t secure the higher accuracy of the water depth measurement. For example, if Ho=2000 Cm, hx=1000 Cm, xcex94h=100 Cm, t=xe2x88x9210xc2x0 C., measuring the water depth hx (=1000 cm) is as follows:                     Δ        p2            γ        =                  -        31            ⁢              xe2x80x83            ⁢      cm        ,      
    ⁢                    Δ        px            γ        =                  -        34            ⁢              xe2x80x83            ⁢      cm        ,      
    ⁢                              P          ax                -                              γ            gx                    ⁢                      H            o                              γ        =                                        -            3.6                    ⁢                      xe2x80x83                    ⁢          cm                ⁢                  
                ∴                  Δ          ⁢                      xe2x80x83                    ⁢                      h            x            xe2x80x2                              =                                    -            34                    +                                    (                                                -                  31                                +                34                            )                        ⁢                          10              1                                -          36                =                              -            7.6                    ⁢                      xe2x80x83                    ⁢          cm                    
As a result, a water column pressure measuring error becomes larger, if a water depth in a reservoir, a lake, a river, etc. is measured by a water level meter, because an error of a pressure transducer that measures a bubble pressure is greatly changed dependent upon a circumferential air temperature t and a pressure P to be measured at an unattended observation post under the condition that an atmosphere temperature is in the range of xe2x88x9240xc2x0 C. in Summer to +40xc2x0 C. in Winter.
There often have occurred larger measuring errors of water depth or water level, due to it that a specific gravity of water gravity shifts at any time and an altitude difference between a water surface and a position of the transducer to be mounted, an altitude difference between lower and upper ends of a water column pressure measuring tube and a difference between a pressure of a bubble generating compressed gas to be measured and a pressure at a water depth to be measured based on a compressed density upon the bubble generating lies in a larger range.
As described above, the conventional technology has a disadvantage in that its water level measurement in a reservoir cannot secure the accuracy of xc2x11 cm contrary to the fact that the water level measurement of the reservoir requires the accuracy of less than xc2x11 cm.
For the purpose of explaining the invention in detail, firstly considering the property of a pressure transducer, the common characteristics of pressure transducers broadly well known are as follows:
The pressure transducer includes a temperature characteristic compensation device, because the property of a pressure sensor is changed according to a temperature. But, it generally doesn""t compensate for the temperature. If a circumferential temperature is beside a normal one, for example the temperature 20xcx9c25xc2x0 C., the pressure measuring error is increased.
For example, the pressure transducers Model PTX1000 and PDCR882 having a relatively good performance has the following characteristics: when the rom temperature t=24xc2x0 C., the Model PTX1000 represents that xcex4=xc2x10.25% fs, the Model PDCR862 represents that xcex4=xc2x10.1%fs. The usable temperature ranges of each of Models PTX1000 and PDCR862 are represented in table 1 as xe2x88x9240xc2x0 C.xcx9c+90xc2x0 C. and xe2x88x9254xc2x0 C.xcx9c+125xc2x0 C. Tables 1 and 2 represents results that a pressure error xcex94p is calculated into a water column pressure cmH2O, in which the pressure error xcex94p is a difference between a reference pressure based on an outputting signal, when the temperature t is 24xc2x0 C., and a pressure based on an outputting signal, when the temperature t is changed in the range of xe2x88x9220xc2x0 C.xcx9c+50xc2x0 C. the pressure difference xcex94p is equal to an absolute error of a water depth measurement, if a water specific gravity xcex3=1.0. Relative errors xcex4% are represented under the pressure error xcex94p. (Calculated data is rounded to the two decimals.)
Changing only the temperature of compressed air, but not cooling or freezing the pressure transducer on the whole, means that the pressure transducers are represented as better characteristics than those of Table 1. But, if the pressure transducer is used in a bubble water level meter, its whole temperature is changed according to a circumferential temperature. Table I represents characteristics when a pressure is increased from 0 to 1 kg/cm2, in which the characteristics are better than when a pressure is reduced from 1 to 0 kg/cm2. (It means Histeresis error or characteristics.)
Table 2 represents the characteristics of the pressure transducer Model PDCR826 that is better than those of Model PTX1000. Seeing Table 2, the temperature compensation error is little, when the temperature becomes +40, +30 or +10xc2x0 C. But, the temperature compensation error is relatively larger, when the temperature t becomes 0, xe2x88x9210, xe2x88x9220 or +50xc2x0 C. the Histeresis characteristics of the Model PDCR826 is inferior to that of Model PTX1000 the pressure error xcex94p is up to two times over Table 2, if the pressure is shifted from a high state to a low state. if the Histeriesis characteristics is bad, the pressure error becomes greater, when the water depth is small.
Seeing the temperature compensation characteristics, when a maximum pressure is applied, the pressure transducer is adjustable near to an error xcex4%fs at a normal temperature with a relative error being less. The less a measuring pressure, the greater an error xcex4 is.
FIG. 3 shows a changing curve of a pressure error 4p when t=0xc2x0 C. and xe2x88x9210xc2x0 C., from which the following characteristics are known. When P=0, the output of Model PTX1000 becomes zero. The reason is because a temperature compensation circuit itself mounted in the pressure transducer represents a zero deviation according to the temperature change. If only the temperature of the compressed air is changed, but the temperature compensation circuit is not heated or cooled, the curve of 4p=f(t,P) is raised to about 26 cmH20, and the pressure error Op becomes less, significantly.
But, the bubble type water level meter is heated or cooled on the whole in seasons, if it is mounted a water level observatory post that is not equipped with cooling and heating devices. Therefore, a water column pressure measuring error is changed due to the pressure error characteristics xcex94p=f(t;P) changed according to the temperature t and measuring pressure P in Tables 1 and 2. The curve of xcex94p=f(t;P) is divided into several sections to be represented as an approximate straight line in each section.
An object of the invention is to measure a water depth in an accuracy using water column pressure measuring tubes of nxe2x89xa72 under the condition that a circumferential temperature t is changed in the range of xe2x88x9240xc2x0 C.xcx9c+50xc2x0 C., even though the property of a pressure transducer is severely changed.
According to the invention, a water depth measuring method comprises steps of making a bundle of reference water column pressure measuring tubes of a U character form of the number of nxe2x89xa72 at a same length with a water depth measuring tube in order to compensate for a total error xcexa3xcex94x of a water depth measurement; filling an amount of water into the reference water column pressure measuring tubes to form water columns ho1, ho2, . . . hon therein; measuring the water pressure in the reference water column pressure measuring tube by a pressure transducer before measuring a water depth; subtracting previously known water column pressure xcex3hoi and xcex3hoj derived from the measured water pressure values Pmi, and Pmj to calculate total errors xcexa3xcex94i and xcexa3xcex94j; calculating a total error xcexa3xcex94x based on the total errors xcexa3xcex94i and xcexa3xcex94j, in which the total error xcexa3xcex94x was generated upon measuring of the bubble generating pressure Pmx in the water column pressure measuring tubes; subtracting the total error xcexa3xcex94x from the bubble generating pressure Pmx, dividing the calculated water column pressures by the water column pressure xcex3hoi(j) in the reference water column pressure measuring tube having most approximate value thereto and multiplying the re-calculated value by the water column pressure hoi(j) thereby to measure a water depth in a higher accuracy.
A bubble water depth measuring system comprises a compressed gas generator, a pressure transducer, a reference water column pressure measuring tube, a water depth measuring tube, an arithmetic logical transducer for calculating a water depth and a drive controller for supplying/interrupting compressed gas.
The water depth measuring tube has a length corresponding to a water depth. The reference water column pressure measuring tubes of nxe2x89xa72 of a U shape has the same inner diameter as that of the water depth measuring tube coupled therewith. A diaphragm is connected through an electromagnetic valve to one tube of the reference water column pressure measuring tube to adjust the speed of the compressed gas from a buffer tank to be supplied to the reference water column pressure measuring tubes. A transparent container is connected to the other tube of the reference water column pressure measuring tubes to measure an amount of water therein and check it, periodically, and includes a nipper for supplementing water reduced due to the evaporation. A tube is directly mounted above the transparent container and has the same inner diameter as that of the reference water column pressure measuring tubes and a length portion, on the surface of which scales are formed. A thin film tube is made of rubber and directed to the upper portion of the tube to be expanded by a gas pressure, and a manual valve connected to the upper end of the thin film tube.